Memory cell layout for low current field-induced MRAM

ABSTRACT

Embodiments of the present invention disclose an MRAM cell layout for 32 nm, 45 nm, and 65 nm CMOS process technology.

This application is a continuation of U.S. application Ser. No. 14/619,701 filed on Feb. 11, 2015, which is a division of U.S. patent application Ser. No. 13/369,267, filed Feb. 8, 2012, which claims the benefit of priority to U.S. Provisional Patent Application No. 61/440,630, filed Feb. 8, 2011, for which the entire specification and drawings of each application is incorporated here by reference.

FIELD

Embodiments of the invention relate to low power field-induced MRAM cell layouts for 65 nm, 45 nm and 32 nm CMOS nodes.

BACKGROUND

In a field-induced magnetic random access memory (MRAM) a current-induced magnetic field generated around metal lines is used to write data into memory cells. Each memory cell stores a bit of data in a magnetic tunnel junction (MTJ). The MTJ is located at the intersection of two conductors in the form of a bit line and a word line. Normally, these lines are laid out perpendicular to each other. To write binary data (a “0” or a “1”) to a MTJ cell, enough current must flow simultaneously through the bit line and the word line associated with that particular cell for a certain amount of time. The sense in which the current flows in both metal lines sets the data value “0” or “1” in the cell.

In some embodiments of MRAM, data may be read from an MTJ through an access transistor connected to the MTJ, which forms part of the memory cell. This transistor is unique to the MTJ being addressed but parts of the transistor may be shared with transistors from neighboring cells.

SUMMARY

Embodiments of the present invention disclose an MRAM cell layout for 32 nm, 45 nm, and 65 nm CMOS process technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing Level 1 of a MRAM cell layout, in accordance with one embodiment of the invention.

FIG. 2 is a schematic drawing showing Level 2 of the MRAM cell layout, in accordance with one embodiment of the invention.

FIG. 3 is a schematic drawing showing Level 3 of the MRAM cell layout, in accordance with one embodiment of the invention.

FIG. 4 is a schematic drawing showing a combination of Levels 1-3 of the MRAM cell layout.

FIG. 5 is a schematic drawing of a MRAM cell based on a MRAM cell layout, in accordance with one embodiment of the invention

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structures and devices are shown in block diagram form only in order to avoid obscuring the invention.

Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.

Although the following description contains many specifics for the purposes of illustration, anyone skilled in the art will appreciate that many variations and/or alterations to said details are within the scope of the present invention. Similarly, although many of the features of the present invention are described in terms of each other, or in conjunction with each other, one skilled in the art will appreciate that many of these features can be provided independently of other features. Accordingly, this description of the invention is set forth without any loss of generality to, and without imposing limitations upon, the invention.

Field-induced MRAM relies on a magnetic field generated around metal lines to switch the magnetization of a MTJ. In most embodiments each cell has two crossing metal lines for this purpose. These lines are the bit line and the word line, respectively. The MTJ is located at the intersection of these two lines and at least the bit line is in contact with the MTJ.

In some embodiments of MRAM, the memory cell also comprises an access transistor connected to the MTJ. The final cell size may be defined by the metal lines' pitch and/or by the access transistor size.

Embodiments of the present invention disclose different MRAM cell layouts, and MRAM structures (cell and devices) based on said cell layouts. Each MRAM cell layout is optimized for a given level of CMOS process technology.

For clarity, the MRAM cell layout is divided into three levels. FIG. 1 shows the first level, FIG. 2 shows the second level, and FIG. 3 shows the third level. In the figures, M1 refers to a first level of metallization, M2 a second level, and M3 a third level.

FIG. 1, shows the layout for two cells at the CMOS transistor level, in plan view. Referring to FIG. 1, reference numeral 1 refers to a gate formed between contacts 2. The gate may be of poly-silicon, in one embodiment. Reference numeral 3 indicates an active area for the MRAM cell. Reference numeral 4 indicates the cell border. Reference numeral 5 indicates a metal 1 (M1) island. Reference numeral 6 indicates a via, referred to as via 1. Reference numeral 7 indicates a metal 1 line. Reference numeral 8 indicates a MTJ. Reference numeral 9 indicates a bottom electrode. Reference numerals 10 and 11 indicate a metal 2 line and a metal 3 line, respectively. The via 1 connects M2 and M1.

The second level depicted in FIG. 2 shows the MRAM cell layout from the M1 layer up to via 1, in plan view. Metal line 7 connecting the source is common for both transistors (left and right).

Referring to FIG. 3, the third level shows the magnetic region of the cell in plan view. The magnetic region includes layers: bottom electrode (9), MTJ (8), metal 2 (10) and metal 3 (11). The bottom electrode is also a seed layer for the MTJ and connects the MTJ (8) to via 1 (6), which is the element connected to the access transistor through M2 (10) and Interface via (12). This electrode is shaped so that the MTJ (8) can be placed at the center of the cell, as shown in FIG. 3. In this way the magnetic interaction between neighboring cells is minimized, which is important to reduce variation in the switching field of the MTJ's.

FIG. 4 shows all levels of the cell combined.

FIG. 5 shows a side view of the MRAM cell, fabricated in accordance with the MRAM cell layout described above.

The final cell size depends on available CMOS manufacturing process capability. According to current manufacturing process a list of dimensions of the different elements of the cell and distances between some of those elements (as numbered in FIGS. 1 to 4) is provided in Table 1. The table covers MRAM cell layouts for CMOS nodes 65 nm, 45 nm and 32 nm.

TABLE 1 MRAM Cell Layout for 32 nm CMOS process technology Size (nm) 32 nm node Element Description cell 1 Po1y-Si gate 18 2 Contact 54 × 54 3 Active area 70 width 4 Cell border 180 × 140 5 Metal 1 island 67 × 84 6 Via 1 54 × 54 7 Metal 1 line 58 8 MTJ 46 × 46 9 Bottom 95 × 66 electrode 10 Metal 2 line 58 12 Interface via 54 × 54 11 Metal 3 line 58 d1 35 d2 114  d3 21 d4 21 d5 56 d6 114  d7 90 d8 57

TABLE 2 MRAM Cell Layout for 45 nm CMOS process technology Size (nm) 45 nm node Element Description cell 1 Poly-Si gate 25 2 Contact 70 × 70 3 Active area 80 width 4 Cell border 250 × 160 5 Metal 1 island 90 × 80 6 Via 1 70 × 70 7 Metal 1 line 80 8 MTJ 65 × 65 9 Bottom 125 × 80  electrode 10 Metal 2 line 80 12 Interface via 70 × 70 11 Metal 3 line 80 d1 40 d2 160  d3 35 d4 30 d5 80 d6 160  d7 125  d8 85

TABLE 3 MRAM Cell Layout for 65 nm CMOS process technology Size (nm) 65 nm Element Description node cell 1 Poly-Si gate 35 2 Contact 100 × 100 3 Active area 110 width 4 Cell border 350 × 220 5 Metal 1 island 130 × 110 6 Via 1 100 × 100 7 Metal 1 line 110 8 MTJ 100 × 100 9 Bottom 195 × 130 electrode 10 Metal 2 line 120 12 Interface via 100 × 100 11 Metal 3 line 20 d1 55 d2 235 d3 50 d4 50 d5 110 d6 230 d7 175 d8 110

In one embodiment, the MRAM cell layouts specified above may be stored in a format that supports data exchange of integrated circuit layouts. For example, the MRAM cell layouts may be stored in a Graphic Database System (GDS) format such as in GDSII format stored a computer-readable medium. Examples of computer-readable media include but are not limited to recordable type media such as volatile and non-volatile memory devices, floppy and other removable disks, hard disk drives, optical disks (e.g., Compact Disk Read-Only Memory (CD ROMS), Digital Versatile Disks, (DVDs), etc.).

Although the present invention has been described with reference to specific example embodiments, it will be evident that various modifications and changes can be made to these embodiments without departing from the broader spirit of the invention. 

The invention claimed is:
 1. A magnetic random access memory (MRAM) circuit comprising: first and second cells, each comprising: a cell border defining a horizontal outer perimeter of a layout of the MRAM circuit; a horizontal plane center within the cell border; a bottom electrode located entirely within the cell border; a magnetic tunnel junction (MTJ) structure located in the cell border and above the bottom electrode; and an access transistor including a first terminal, a second terminal, and a gate terminal, wherein: the MTJ structure is centered within the cell border with respect to the horizontal plane center; the bottom electrode comprises a shape enabling the MTJ structure to be centered within the cell border with respect to the horizontal plan center; and the bottom electrode is not centered at the horizontal plane center, wherein the horizontal plane centers corresponding to each of the first and second cells are separated by a distance equal to the length of the cell border to minimize magnetic interactions between the MJT structure corresponding to each of the first and second cells.
 2. The MRAM circuit of claim 1, wherein a ratio of a length of the bottom electrode along an axis to a length of the MTJ structure along the same axis is equal to or greater than 1.9.
 3. The MRAM circuit of claim 2, wherein the ratio is 2.1 when the MRAM circuit is fabricated at a 32 nm complementary metal-oxide semiconductor (CMOS) process technology.
 4. The MRAM circuit of claim 2, wherein the ratio is 1.9 when the MRAM circuit is fabricated at a 45 nm CMOS process technology.
 5. The MRAM circuit of claim 2, wherein the ratio is 2.0 when the MRAM circuit is fabricated at a 65 nm CMOS process technology.
 6. The MRAM circuit of claim 1, wherein the MTJ structure being centered within the cell border minimizes magnetic interactions between one or more neighboring cells of the cell.
 7. The MRAM circuit of claim 1, wherein the MTJ structure being centered within the cell border reduces a variation in a switching field of the MTJ structure.
 8. The MRAM circuit of claim 1, wherein a size of the MTJ when the MRAM circuit is fabricated is equal to or greater than 45 nm by 45 nm.
 9. The MRAM circuit of claim 1, wherein the horizontal plane center divides a length of the cell border in half.
 10. The MRAM circuit of claim 1, wherein the shape of the bottom electrode comprises a length spanning from a location of an interface via to the horizontal plane center.
 11. The MRAM circuit of claim 10, wherein the location of the interface via being closer to the cell border than to the horizontal plane center.
 12. The MRAM circuit of claim 1, wherein a size of the MRAM circuit comprises a cell border equal to or greater than 180 nm by 140 nm.
 13. The MRAM circuit of claim 1, wherein a size of the MRAM circuit comprises a cell border with a length to width ratio equal to or greater than 0.6.
 14. The MRAM circuit of claim 1, wherein the minimizing of the magnetic interactions reduces variation in switching fields of the MTJ structures of the first and second cells.
 15. A magnetic random access memory (MRAM) circuit comprising: first and second cells, each comprising: a cell border defining a horizontal outer perimeter of a layout of the MRAM circuit; a horizontal plane center within the cell border; a bottom electrode located entirely within the cell border; a magnetic tunnel junction (MTJ) structure located in the cell border and above the bottom electrode; and an access transistor including a first terminal, a second terminal, and a gate terminal, wherein: the MTJ structure is centered within the cell border with respect to the horizontal plane center; the bottom electrode comprises a shape enabling the MTJ structure to be centered within the cell border with respect to the horizontal plane center; the bottom electrode is not centered at the horizontal plane center; and a ratio of a length of the bottom electrode along an axis to a length of the MTJ structure along the same axis is equal of greater than 1.9, wherein the horizontal plane centers corresponding to each of the first and second cells are separated by a distance equal to the length of the cell border to minimize magnetic interactions between the MJT structure corresponding to each of the first and second cells.
 16. The MRAM circuit of claim 15, wherein the ratio is 2.1 when the MRAM circuit is fabricated at a 32 nm complementary metal-oxide semiconductor (CMOS) process technology.
 17. The MRAM circuit of claim 15, wherein the ratio is 1.9 when the MRAM circuit is fabricated at a 45 nm CMOS process technology.
 18. The MRAM circuit of claim 15, wherein the ratio is 2.0 when the MRAM circuit is fabricated at a 65 nm CMOS process technology.
 19. A magnetic random access memory (MRAM) circuit comprising: a plurality of cells comprising at least first and second cells, each cell comprising a cell border and a horizontal plane center within the cell border, the cell border defining a horizontal outer perimeter of a layout of the cell, each cell comprising: a first level; a second level between the first and second level; and a third level comprising an interface via, an electrode, a magnetic tunnel junction (MTJ) structure, and a metal, wherein a shape of a bottom electrode of the cell comprises a length spanning from a location of the interface via to the horizontal plane center of the cell, wherein the MJT structure is located between the bottom electrode and a third metal at the horizontal plane center to minimize magnetic interactions between other cells of the plurality of cells, wherein the horizontal plane centers corresponding to each of the first and second cells are separated by a distance equal to the length of the cell border to minimize magnetic interactions between the MJT structure corresponding to each of the first and second cells.
 20. The MRAM circuit of claim 19, wherein the horizontal plane center divides a length of the cell border in half. 